The present invention relates to a heater capable of raising the temperature of a heating element portion thereof through supply of electricity to the portion, such as a glow plug used in an internal combustion engine for improving start-up of the engine, and to a method for manufacturing the same.
In order to improve the start-up of a diesel engine at low temperature, the heating element of a glow plug is disposed within the combustion chamber. Applying electricity to the glow plug heats the heating element and accelerates ignition of fuel, thereby enhancing start-up.
In some cases, in order to heat a liquid such as cooling water or a gas such as air in an engine, a glow plug may be used as a heater. Similarly, a heater having a similar configuration may be used as a heat source for igniting kerosene or a gas.
A glow plug is generally configured in the following manner: a heating element is disposed in a cylindrical metallic shell in such a manner as to project from the front end of the metallic shell. One electrode of the heating element is electrically connected to the metallic shell while the other electrode is electrically led to an external terminal, which is disposed in the vicinity of the rear end of the metallic shell while being electrically insulated from the metallic shell, by use of a rod-like axial member, a lead wire, or other electrically conductive member.
However, in an engine, since the heating element of a glow plug is disposed within a combustion chamber or a prechamber, which is exposed to high pressure, the glow plug must be gas-tight such that a gas within the combustion chamber does not leak through the glow plug (through the metallic shell) to the exterior of the glow plug.
When a heating element is configured such that a heating resistor, formed of a high-melting-point metal wire, together with a ceramic powder heat resistant insulation, such as MgO, is disposed within a closed-bottomed cylindrical metal sheath, the glow plug must also be gas-tight. This prevents the ceramic powder insulation from absorbing moisture and deteriorating in insulating performance, from entry of water, water vapor, or oil from the side toward the external terminal (the side toward the rear end of the metallic shell).
Also, a heater that serves as an ignition heat source for heating water or the like must be gas-tight so as to prevent leakage of water, water vapor, or the like to the exterior of the heater or entry of the same into the heater, through the metallic shell.
In order to establish such gas-tightness, a glow plug or a like heater employs a seal mechanism, such as a glass seal or an O-ring, provided in the vicinity of the rear end portion of the metallic shell. However, employment of a seal mechanism such as a glass seal or an O-ring involves various problems such as an increased number of manufacturing steps, resulting in increased cost.
The present invention has been accomplished in view of the above-mentioned problems, and an object of the invention is to provide an inexpensive heater with good gas-tightness, as well as a method for manufacturing the same.
The present invention is a heater comprising a cylindrical metallic shell having a front end, a rear end, and a through-hole extending therein between the front end and the rear end. A heating element is disposed in the through-hole of the metallic shell such that a portion thereof projects from the front end of the metallic shell. The heating element is adapted to generate heat upon application of electricity thereto. A lead member extends through the through-hole at least from the rear end of the metallic shell while being electrically insulated from the metallic shell, and electrically connected to the heating element. A gas-tight seal member, formed of an insulating polymeric material, is interposed between the lead member and an inner wall surface of the through-hole of the metallic shell in such a manner as to surround at least a longitudinal portion of the lead member. The metallic shell includes a crimped portion at which the metallic shell is crimped from an outer surface thereof to bring the gas-tight seal member into close contact with the lead member and the inner wall surface of the through-hole. This maintains gas-tightness within the through-hole between the side toward the front end and the side toward the rear end with respect to the gas-tight seal member.
In the heater of the present invention, the metallic shell includes a crimped portion at which the gas-tight seal member is in close contact with the lead member and the inner wall surface of the through-hole, to thereby maintain gas-tightness between the side toward the front end and the side toward the rear end with respect to the gas-tight seal member.
Thus, when this heater is used as a glow plug, leakage of high-pressure gas within the combustion chamber of an engine from the side toward the front end to the side toward the rear end can be prevented. Also, entry of water, such as water vapor, or oil from the side toward the rear end to the side toward the front end can be prevented, thereby preventing deterioration of the heat resistant insulation powder within the heating element.
The heater of the invention can establish gas-tightness without provision of a seal mechanism, such as a glass seal or an O-ring, at a rear end portion of the metallic shell, and is therefore inexpensive.
Examples of a heater to which the present invention is applicable include a glow plug used in a diesel engine for assisting start-up, and a heater used as a heat source for heating a liquid such as water or a gas such as air, or for igniting kerosene or the like.
Preferably, the present invention is applied to a heater to be used as a glow plug. That is, preferably, a glow plug comprises a cylindrical metallic shell having a front end, a rear end, and a through-hole extending therein between the front end and the rear end. A heating element is disposed in the through-hole of the metallic shell such that a portion thereof projects from the front end of the metallic shell. The heater portion is adapted to generate heat upon application of electricity thereto. A lead member extends through the through-hole, at least from the rear end of the metallic shell, while being electrically insulated from the metallic shell. The lead member is electrically connected to the heating element. A gas-tight seal member, formed of an insulating polymeric material is interposed between the lead member and an inner wall surface of the through-hole of the metallic shell in such a manner as to surround at least a longitudinal portion of the lead member. In the glow plug, the metallic shell includes a crimped portion at which the metallic shell is crimped from an outer surface thereof so as to bring the gas-tight seal member into close contact with the lead member and the inner wall surface of the through-hole, to maintain gas-tightness within the through-hole between the side toward the front end and the side toward the rear end with respect to the gas-tight seal member.
Preferably, the heater of the present is gas-tight such that no leakage arises in the course of a gas-tightness test conducted through application of a gas pressure of 1.5 MPa to the gas-tight seal member from the side toward the front end.
The heater of the present invention has high gas-tightness such that no leakage arises even when high gas pressure is imposed thereon. Thus, gas-tightness can be reliably maintained between the side toward the front end and the side toward the rear end with respect to the gas-tight seal member.
Having such high gas-tightness, the heater used as a glow plug exhibits high reliability.
Preferably, the above-described heater is configured such that a total contact area S between the gas-tight seal member and the inner wall surface of the through-hole as measured in a region located radially inward of the crimped portion is not less than 45 mm2.
In this heater, the gas-tight seal member has a predetermined total contact area, as measured inside the crimped portion, such that it can remain gas tight when exposed to the application of a gas pressure of 1.5 MPa.
Preferably, any one of the above-described heaters is configured such that the lead member comprises a rod-like axial member and a connection member for electrically connecting the front end portion of the lead member and the heating element, and the gas-tight seal member is interposed between the axial member and the inner wall surface of the through-hole of the metallic shell in such a manner as to surround at least a certain longitudinal portion of the axial member.
In the heater of the present invention, since the lead member includes the rod-like axial member, as compared with the case of using a fine wire in place of the axial member, electrical resistance can be reduced, and the area of contact with the gas-tight seal member can be increased. Accordingly, it becomes difficult to axially draw the axial member from the gas-tight seal member; i.e., the axial member and the gas-tight seal member are joined with high strength, and thus the axial member and the metallic shell are joined strongly via the gas-tight seal member.
Use of this heater as a glow plug is particularly preferred, for the following reason. Since the axial member can be fixedly attached to the metallic shell via the gas-tight seal member, even when the axial member is subjected to vibration associated with engine operation, free vibration of the axial member can be prevented, thereby enhancing durability of the glow plug.
Preferably, the above-described heater is configured such that an outer circumferential surface of the axial member to be covered with the gas-tight seal member is at least partially roughened.
In the heater of the present invention, since a portion of the outer circumferential surface of the axial member is roughened, good adhesion is attained between the gas-tight seal member and the outer circumferential surface of the axial member, thereby enhancing gas-tightness of the heater. Also, the axial member becomes unlikely to axially come off the gas-tight seal member; i.e., the metallic shell.
No particular limitation is imposed on a roughening process, so long as the outer circumferential surface of the axial member is roughened. Examples of such a roughening process include a mechanical roughening process such as knurling, sandpapering, or sandblasting, and a chemical roughening process.
Preferably, at least an inner wall surface of the through-hole of the metallic shell to be covered with the gas-tight seal member is at least partially roughened.
Such roughening establishes good adhesion between the gas-tight seal member and the inner wall surface of the through-hole of the metallic shell, thereby further enhancing gas-tightness of the heater. Also, the axial member becomes unlikely to axially come off the gas-tight seal member; i.e., the metallic shell.
When the axial member projects from the rear end of the metallic shell so as to serve as an external terminal, or when the axial member is fixedly attached to an external terminal in the vicinity of the rear end of the metallic shell, it is preferred that the outer circumferential surface of the axial member to be covered with the gas-tight seal member is at least partially roughened as described above.
Since a connection terminal of a power cord is fixedly or removably attached to the external terminal, the external terminal must be fixedly attached to the metallic shell so as not to be extracted along the axial direction. When the axial member is used as an external terminal or when the axial member is fixedly attached to an external terminal, roughening the surface of the axial member as described above allows the axial member to be reliably fixed to the metallic shell.
Preferably, any one of the above-described heaters is configured such that the gas-tight seal member has a Vickers hardness HV of 10-80 as measured at a position located radially inward of the crimped portion. This affixes the axial member within the seal member such that it takes a tensile force of not less than 2,000 N to extract the axial member from the seal member.
More preferably, the gas-tight seal member has a Vickers hardness HV of 20-80, for the following reason. When the hardness HV is less than 20, for example, the gas-tight seal member is prone to deformation during the course of a tensile test on the axial member. Therefore, in order to enhance the tensile strength of the axial member for stronger fixation of the axial member, the length of a crimped portion must be increased.
Still more preferably, the gas-tight seal member has a Vickers hardness HV of 20-60, for the following reason. When the hardness HV exceeds 60, there is a possibility that the gas-tight seal member may be cracked in the course of crimping.
The gas-tight seal member is preferably formed of a thermoplastic resin, for the following reason. By employment of thermoplastic resin, the gas-tight seal member can be readily formed on the lead member such as the axial member through injection molding or a like process.
Also, the gas-tight seal member is preferably formed of a heat-resistant polymeric material; specifically, a polymeric material having a melting point not lower than 200xc2x0 C. Specific examples of such a polymeric material include polyether ether ketone (PEEK) and polyphthalamide (PPA). Such polymeric materials are preferred, for the following reason. When the heater is used as a glow plug, the gas-tight seal member is possibly exposed to a high temperature of at least 150xc2x0 C., although the temperature depends on the position of the gas-tight seal member and specifications of an engine.
Another embodiment of the invention is a method for manufacturing a heater. The method includes the step of disposing a heating-element-lead-member assembly in a through-hole of a cylindrical metallic shell having a front end, a rear end, with the through-hole extending therein between the front end and the rear end, such that a portion of the heating element projects from the front end, and the lead member extends to the rear end. The heating-element-lead-member assembly comprises a heating element adapted to generate heat upon application of electricity thereto, and a lead member electrically connected to the heating element and including a gas-tight seal member. The gas-tight seal member surrounds at least a longitudinal portion of the lead member and is formed of an insulating polymeric material. The method further includes crimping the metallic shell from an outer surface thereof so as to bring the gas-tight seal member into close contact with the lead member and an inner wall surface of the through-hole, to thereby maintain gas-tightness within the through-hole between a side toward the front end and a side toward the rear end with respect to the gas-tight seal member.
According to the method for manufacturing a heater of the present invention, the gas-tight seal member is formed of an insulating polymeric material beforehand in such a manner as to surround at least a longitudinal portion of the lead member, and the resultant assembly is disposed within the through-hole of the metallic shell in the disposing step. Thus, the disposing step can be readily carried out merely through insertion of the heating-element-lead-member assembly into the through-hole of the metallic shell. Also, the gas-tight seal member can be disposed at a predetermined position without need to perform a particular positioning operation.
In another aspect, a method is provided for manufacturing a heater. The method includes the step of disposing a heating-element-axial-member assembly in a through-hole of a cylindrical metallic shell having a front end, a rear end, and the through-hole extending therein between the front end and the rear end, such that a portion of the heating element projects from the front end. The heating-element-axial-member assembly comprises a heating element, adapted to generate heat upon application of electricity thereto, an axial member formed of a metal and including a gas-tight seal member, and a connection member for electrically connecting the heating element and a front end portion of the axial member, the gas-tight seal member surrounding at least a longitudinal portion of the axial member and being formed of an insulating polymeric material. The method includes the step of crimping the metallic shell from an outer surface thereof so as to bring the gas-tight seal member into close contact with the axial member and an inner wall surface of the through-hole, to thereby maintain gas-tightness within the through-hole between a side toward the front end and a side toward the rear end with respect to the gas-tight seal member.
According to the method for manufacturing a heater of the present invention, the gas-tight seal member formed of an insulating polymeric material is formed beforehand in such a manner as to surround at least a certain longitudinal portion of the axial member, and the resultant assembly is disposed within the through-hole of the metallic shell in the disposing step. Thus, the disposing step can be readily carried out merely through insertion of the heating-element-axial-member assembly into the through-hole of the metallic shell. Also, the gas-tight seal member can be disposed at a predetermined position without need to perform a particular positioning operation.